Known computed tomography (CT) imaging systems include a rotatable annular gantry or rotor having a gantry bore for receiving a patient or other object to be internally imaged. In typical CT applications, an x-ray source is mounted on the annular gantry and radiates a fan-shaped, wedge-shaped, or cone-shaped x-ray beam in the plane of gantry rotation. The x-ray beam passes through the patient while the annular gantry is rotating and the attenuated x-rays are sensed by a detector array disposed on the gantry opposite to the x-ray source.
Thus, while the annular gantry is rotating, a series of x-ray projections or 2-D slices of the patient are obtained at different angles. These projections are mathematically reconstructed to create a tomographic image of each slice. The patient may be moved axially through the bore to obtain data on adjacent slices and the slices may be combined to generate a 3-D image of interest.
Generally speaking, higher rotational speeds are desired in order to reduce the time required to obtain the tomographic image of interest and/or to provide high-speed or “freeze motion” images. However, higher rotational speeds often lead to problems with respect to the static and dynamic balance of the rotating annular gantry.
In the past, it has been attempted to maintain the balance of the gantry by setting very tight tolerances for the center of gravity and the masses of the components mounted on the gantry. These components generally include the x-ray source and detector, signal processing circuitry, power supplies and cooling systems. The gantry may then be manually balanced by adding and adjusting balancing weights, which is a time consuming and difficult task.
Thus, the need to dynamically offset any imbalances arising during operation has required relatively complex solutions, such as the solution described in U.S. Pat. No. 6,748,806, which changes the position of movable weights disposed on the annular gantry during operation to maintain the dynamic balance of the annular gantry.
Further, if it becomes necessary to replace or service the mechanical bearings supporting the gantry, subsequent field rebalancing is more difficult than balancing at the time of manufacturing and also leads to a loss of valuable usage time for the system. In addition, the re-application of lubricants, e.g., bearing grease, in a sterile environment such as a medical facility is also problematic.
U.S. Pat. No. 7,277,523 mentions the use of high speed mechanical bearings, air bearings, magnetic beatings, or the like to rotatably support the annular gantry, but provides no details concerning the construction of the bearings. U.S. Pat. No. 7,023,952 discloses air bearings for rotatably supporting the annular gantry of a diagnostic scanning device.
WO 2010/026523 A2 discloses a rotating ring apparatus that includes magnetic levitation means for levitating and rotating a rotating ring relative to a stationary ring.
U.S. Pat. No. 5,481,585 discloses a CT machine that utilizes magnetic levitation magnets to levitate a rotor, which is rotated about its rotational axis using a separate drive means.
US 2011/0194669 discloses a diagnostic scanning apparatus, such as a CT machine, that uses a magnetic bearing system and provides a significant improvement over the prior art. However, further improvements are possible.